The present invention relates to pressure sensitive adhesives based on aqueous latex emulsions and processes for the preparation of the adhesives. Pressure sensitive adhesives prepared according to the present invention have mean particle diameter sizes of less than or equal to about 100 nm and narrow particle size distributions. These pressure sensitive adhesives are particularly suitable for applications that require that the pressure sensitive adhesive maintain adhesion between the substrate and facestock when subjected to hot water spraying or immersion. In addition, the adhesives exhibit resistance to water-whitening or xe2x80x9cblushxe2x80x9d. Hot water adhesion is required in applications such as bottle labels where the bottles are subjected to hot water spraying in washing operations. In general, resistance to water-whitening is desirable anywhere a pressure sensitive adhesive with transparent facestock or substrate is subjected to water or high humidity. Examples include labels on the sides of trucks, signs and bottles.
Methods of providing water-whitening resistant latex emulsions for use in pressure sensitive adhesives are disclosed in the art. U.S. Pat. Nos. 5,286,843 and 5,536,811 disclose a process for improving the water-whitening resistance of pressure sensitive adhesives containing an aqueous latex emulsion and water soluble ions by removing the water soluble ions and adjusting the pH to at least about 6. The patents disclose that water soluble ions may be removed by a number of techniques including centrifugation, dialysis, precipitation and deionization with ion exchange resins. The preferred method of removing the water soluble ions is to contact the aqueous latex emulsion, the formulated pressure sensitive adhesive containing the aqueous emulsion or both with an ion exchange resin. International Application WO 97/11996 discloses a process for preparing hot water-whitening resistant latex emulsions useful in pressure sensitive adhesive compositions. The process involves copolymerizing a monomer mixture containing at least one alkyl acrylate ester of an alcohol containing at least 4 carbon atoms, at least one polar co-monomer and at least one partially soluble co-monomer present in an amount of at least about 7 weight %. Polymerization is carried out in the presence of at least one nonionic surfactant containing at least 8 moles of ethylene oxide and at least one anionic surfactant containing up to about 10 moles of ethylene oxide. The polymerization product is neutralized to produce an emulsion having a pH greater than 7 and containing particles having a volume average particle size diameter up to about 165 nm. An electrolyte may be added subsequent to polymerization to stabilize opacity of a film cast from the emulsion. International Application WO 98/44064 discloses inherently tacky pressure sensitive adhesives prepared by emulsion polymerization of at least one monomer mixture comprising; at least one alkyl acrylate, the alkyl group of which has from 4 to 12 carbon atoms; at least one unsaturated carboxylic acid containing from about 3 to 5 carbon atoms and one styrenic monomer; wherein the particles have a mean diameter of 300 nm or less. The publication discloses a single stage preparation of aqueous acrylic emulsions in examples 4D, 4E, 4F, 4G and 4H with average particle sizes ranging from 245 nm to 139 nm. Each of the examples discloses the use of silane crosslinkers to improve blush resistance. The reference discloses a preferred method of preparation which yields adhesives resistant to water-whitening and involves a sequential polymerization of a first and second monomer charge. None of the above references disclose a pressure sensitive adhesive that maintains adhesion in hot water environments and is resistant to water-whitening.
It has been found that pressure sensitive adhesives that maintain adhesion in hot water environments in addition to exhibiting enhanced resistance to water-whitening can be prepared using an aqueous latex emulsion prepared from a monomer mixture consisting essentially of;
a) at least one alkylacrylate having at least 4 carbon atoms in the alkyl chain,
b) at least one ethylenically unsaturated carboxylic acid or its corresponding anhydride,
c) at least one styrenic monomer.
The monomer mixtures are polymerized using single stage aqueous latex emulsion techniques. The polymerization is carried out in the presence of at least one anionic surfactant and a redox type free radical initiator system. The present invention further relates to methods of preparation of the pressure sensitive adhesives and laminates prepared using these pressure sensitive adhesives. When prepared using the disclosed monomer mixture, anionic surfactant and redox initiator system, the aqueous latex emulsion has polymer particles with an mean particle size diameter of less than or equal to about 100 nm. In addition, when prepared as described the measured particle size does not increase during the polymerization process at the expected rate, but rather maintains a relatively flat particle growth curve, while still yielding latex emulsions with relatively narrow particle size distributions. Typically, the particle size distribution exhibits a mean/median ratio of less than about 1.05.
Pressure sensitive adhesives of the present invention are useful in clear label applications, marking films, etc. When prepared according to the invention pressure sensitive adhesives maintain adhesion in hot water environments and are resistant to water-whitening for at least 12 hours and preferably for at least 24 hours.